1. Field of the Invention
The present invention relates to an image sensing apparatus, an image sensing system, and an image sensing method, and more particularly to an image sensing apparatus, an image sensing system, and an image sensing method for changing a dynamic range compression characteristic in accordance with a luminance of a subject.
2. Description of the Related Art
The pixel size of an image sensor is being decreased accompanied by a recent demand for a miniaturized and high pixel density image sensor. As a result, the illuminance range (so-called dynamic range) of the image sensor tends to be narrow, which may adversely affect the image quality. In view of this, there is a demand for securing a wide dynamic range of the image sensor.
In an attempt to solve the above drawback, various techniques are proposed. There is disclosed an image sensing apparatus (see e.g. Japanese Examined Patent Publication No. Hei 7-97841) for generating a composite image with a wider dynamic range than the dynamic ranges of individual screen images by sensing the screen images with different exposure amounts a certain number of times, selecting image areas having a proper exposure level from these screen images for image synthesis. There is also disclosed an image sensor (e.g. Japanese Unexamined Patent Publication No. 2000-165755) provided with a charge voltage converter, with plural capacitances having different voltage dependences, for converting a signal charge transferred from a photoelectric converter of the image sensor into a signal voltage to make the dynamic range variable. There is also disclosed a method (see e.g. Japanese Unexamined Patent Publication No. 2000-165754) for extending the dynamic range, with use of an image sensor provided with a number of capacitances for holding signal charges of photodiodes, by reading out signal charges acquired by one-time exposure a certain number of times while switching over the capacitance value, and by implementing summation with respect to the readout signals.
There is also proposed a logarithmic conversion type image sensor (hereinafter, called as “linear-logarithmic sensor”, see e.g. Japanese Unexamined Patent Publication No. 2002-77733) for converting a photocurrent into a logarithmically compressed voltage, using a sub-threshold characteristic of an MOSFET, wherein output characteristics inherent to a solid-state image sensor i.e. a linear conversion operation of linearly converting an electric signal commensurate with an incident light amount for output, and a logarithmic conversion operation of logarithmically converting the electric signal commensurate with the incident light amount for output are automatically switched over by supplying a specific reset voltage to the MOSFET.
Whereas a sensing device such as the linear-logarithmic sensor has succeeded in securing a wide dynamic range, in the current technology, a display device such as a monitor has not succeeded in securing a wide dynamic range, as compared with the sensing device. Even if a wide dynamic range is obtained for an input image, the effect of the wide dynamic range cannot be satisfactorily exhibited on the display device. In other words, the technique of securing the dynamic range in the device for displaying, transferring, or storing an image sensed by the sensing device is limited. Even if a wide dynamic range image is obtained by the various methods as proposed above, it is difficult to process the entirety of the image information obtained by these methods. Accordingly, there is a need for a process of adjusting the dynamic range of an input image for the dynamic range of the device for transferring, storing, or displaying the image, while retaining useful information, relating to the wide dynamic range input image, which has been acquired by the aforementioned methods, for instance, a process (hereinafter, called as “dynamic range compression process”) for compressing the dynamic range of the input image in such a manner that the wide dynamic range input image can be properly displayed with the dynamic range for the display device.
As an example of the dynamic range compression process, there is known a method for creating plural blurred images based on an original image acquired by an image sensor, and creating a composite blurred image based on the obtained blurred images to compress the dynamic range of the original image based on the composite blurred image. For instance, Japanese Unexamined Patent Publication No. 2001-298619 discloses an example of the dynamic range compression approach of varying a processing parameter for creating plural blurred images, or a processing parameter for creating a composite blurred image in compressing the dynamic range in accordance with a scene discrimination result regarding the original image, or photographic information attached to the original image.
The above technology discloses the dynamic range compression method, but does not disclose a gradation conversion method i.e. a contrast correction method, which is a process to be executed independently of the dynamic range compression e.g. a process to be executed after the dynamic range compression. Generally, a conversion process using a gradation conversion characteristic i.e. gamma (γ) is performed as the gradation conversion. Changing the gamma in such a manner as to increase the contrast in a low luminance area so as to optimize the contrast in a main subject area may unduly narrow the gradation in a high luminance area, which may lower the contrast in the high luminance area. The dynamic range compression process including the aforementioned technique has a drawback that a high frequency component i.e. an edge portion is relatively emphasized, because an illumination component primarily including a low frequency component is compressed, thereby generating an unnatural image with a high sharpness i.e. an image with an awkward resolution.